Recently, the railroad industry has shown a high degree of awareness to the problem of missing or broken knuckle pins. They have also given consideration to the cause of failure and any impact the pin may have on causing failure of other coupler parts. The overall results from gathering information and test data lead to the conclusion that pin failure is caused by bending fatigue. Technically, knuckle pins are not to be subjected to bending fatigue inasmuch as the coupler body and knuckle are designed to take the loading resulting from buff and draft forces. Thus, the pins are not designed to take any load. Their design purpose is solely to function as a pivot connection between the knuckle and the coupler body when opening or closing the knuckle. Due to an inability to control dimensional tolerances during casting procedures with common foundry practice of the coupler body and knuckle, the knuckle pin is often subjected to load in both buff and draft actions, thereby resulting in failure due to bending fatigue.
Heretofore, it has been well known to use a steel knuckle pin in coupler assemblies for defining the hinging of the knuckle to the coupler body. It also has been known that this steel pin causes maintenance and operational difficulties such that investigations and studies have been made by the American Association of Railroads. It has caused concern for seeking a solution to pin failure which could lead to derailments and a solution to reducing overall coupler maintenance.
The heretofore known steel pin is susceptible to rust and corrosion problems. Because the pins are sometimes of harder steel than the coupler body and/or knuckle, damage is produced in the coupler body or knuckle requiring replacement of either or both. Although the coupler assembly is constructed with the purpose in mind that the knuckle pin is not subjected to any stress during its use, it is well known that the tolerance of the coupler body and knuckle is such that bending stresses most often are transmitted to the knuckle pin during draft (tension) and buff (compression) of the railway cars.
Failure of a knuckle pin may often go undetected by an inspector because the lower portion of the pin is not readily observable unless the inspector takes time to bend down and inspect the underside of the couplers. Thus, fracture of a pin and the dropping of the lower end of the pin out of the coupler assembly may go unnoticed. This thereafter could cause a coupler failure that could even lead to a serious derailment or other accident. Further, in hopper cars that must be turned over for dumping, fractured or broken pins may fall out of the coupler assembly and enter the crusher, thereby causing damage to the crusher parts. Thus, magnets have been installed in crushers in order to prevent pin segments from going into the crusher. These magnet systems are costly and not totally effective.
While it is desirable to have a steel pin rotate during its use in order to enhance the life of the pin, once a steel pin is bent it cannot thereafter be rotated. A bent pin causes binding between knuckle and coupler body, thereby impeding coupler operations. Further, on open-top hoppers and fixed-end gondolas, the required application of a cotter pin prohibits rotation of the pin. Also, bending fatigue caused by draft and buff operations is the common cause of failure of steel pins. Where pin failures occur in service, such could result in separation of the knuckle from the coupler body which causes undesired emergency stops and possible derailments.
Failure of steel pins is also caused by work-hardening of the pin after it has been bent and kept from free rotation. Some manufacturing difficulties can occur where there is a significant hardness gradient across the diameter of the pin which may have been the result of improper heat treatment procedures.
It is also known that with respect to use of steel pins, since it cannot be determined whether it is broken and where the broken parts are in the coupler assembly, upon opening of the knuckle, the 70 to 80 pound knuckle could fall from the coupler assembly and injure a maintenance worker's foot. Thus, safety of maintenance workers is jeopardized by the use of steel knuckle pins.
It is well known that knuckles were constructed with a hollow area within the body of the knuckle around the knuckle pin hole. One previously known attempt to overcome pin failure was in the design of a straight-through hole in the knuckle and which did produce some favorable effect on the reduction of knuckle pin failures. It has also been proposed to reduce the tolerances for making of coupler bodies and knuckles, but that has not been found to be practical. It has further been proposed to bond a plastic or rubber sleeve to the pin surface to allow the sleeve to deflect easier in the regions of highest stress, but no product of this type has ever been made.